1. Field of the Invention
The present invention relates generally to mining and construction tools and, more particularly, is concerned with a Mn-B steel alloy composition from which to fabricate a mining and construction bit body and with a process of fabricating the body.
2. Description of the Prior Art
Many mining and construction tools employ drums and the like on which are mounted a multiplicity of rotary cutter bits. Typically, each bit has an elongated body which at its forward end has brazed thereon a hard, wear resistant, pointed tip which contacts the formation. Heretofore, hard tips have been composed of any one of several different grades of cemented tungsten carbide composition, whereas bit bodies have typically been fabricated from any one of several standard steel alloys, such as AISI Nos. 4140H, 8630H and 8740H. Representative of the prior art are the cutter bits disclosed in U.S. Pat. Nos. 3,499,685 to Kniff, 3,519,309 to Engle et al, 3,720,273 to McKenry et al, 4,216,832 to Stephenson, 4,316,636 to Taylor et al and 4,497,520 to Ojanen.
The conventional process for fabricating a bit body from one of the standard steel alloys is to start with a rod composed of the steel alloy and having an diamaeter size sufficient to allow machining to the desired final bit body size (the maximum being two inches). Next, the rod is annealed to soften it and thereby facilitate its machinability. Then, the rod is machined to the desired final bit body size and shape. Following next, the bit body is heat treated to obtain the desired mechanical properties of hardness and toughness. The heat treatment includes heating the bit body to a temperature above 1550 degrees F., next, subjecting the bit body to quenching in oil, water or polymer based quenchants to cool and harden it, and, finally, tempering it to improve its toughness. The carbide tip is brazed to the bit body either before or after the heat treatment or concurrently therewith.
In the course of operating mining and construction tools, the bits are forcibly engaged with coal and rock formations to reduce and remove the same and thus are subjected to a high degree of stress and wear. Failure of bit bodies used in mining and construction is due primarily to a bending or breaking moment. Stress produced by the bending moment is at a maximum at the surface of the bit body and decreases to a minimum or zero at its center or axis. Thus, the steel alloys from which the bit bodies are fabricated must have at least a minimum hardenability in order to make the bodies fabricated therefrom capable of withstanding such bending stresses. However, the steel alloys used heretofore that have adequate hardenability properties, must be annealed in order to meet machinability requirements. This necessity for annealing the rods increases the cost of processing the material and ultimately increases the cost of the bit body, for instance by about ten to twenty percent.
Although bit bodies constructed from the standard steel alloys used heretofore have been satisfactory overall, there is a constant need for improvements in the material composing the bit body and the process of fabricating the body in order to further reduce costs but without sacrificing its desired minimum design properties.